Precise metering of liquids and dry solids, such as amorphous powders, is an important but difficult task in many industrial applications and processes. Whenever such materials are continuously fed into a process, the feed rate at which material is delivered to the process must be controlled, just as with any other material used as an ingredient or additive in a continuous or batch process.
Types of material feeding systems include volumetric feeding systems and gravimetric feeding systems. As the name implies, volumetric feeding systems dispense material by volume. They employ a displacement mechanism of some sort (for example, an auger mounted below a supply vessel and feeding a fixed volume of material per auger rotation) operating at a set speed. This results in feeding a known volume of material. The weight of material fed can generally be determined based on the material's bulk density. Volumetric systems cannot always be relied on to produce good results because actual conditions, such as the density, flow and handling characteristics of the material, change during the feeding process.
Where a high degree of accuracy is required, gravimetric feeding systems are employed. These systems monitor their own feeding performance and automatically correct for variations from the desired or set feed rate regardless of product characteristics. Gravimetric systems control the feeding of material by weight, thereby compensating for changes in density and/or irregular product density, flow or handling characteristics. For this reason, these systems can provide much higher accuracy than volumetric feeding systems.
For weigh feeding of materials, there are various types of gravimetric systems used. Two typical types are weigh-belt and loss-in-weight feeding systems. Typical weigh-belt gravimetric feeding systems measure the weight of the material passing across the weigh-belt during operation, that is, while the belt is dispensing material. This measured weight is compared to an expected or "set" weight, causing the generation of a control signal. The control signal either increases, decreases or maintains the speed of the motor driving the belt to achieve a desired feed rate. While such weigh-belt systems offer good accuracy for many uses, material sometimes accumulates on the belt or other critical components and thereby causes the control system to improperly adjust the feed rate. The problem is compounded if the accumulated material builds up and then falls off critical sections of the weighing mechanism causing shifts in the calibration of the scale's "zero" calibration setting. Shifts in the scale's "zero" calibration setting can also be caused by temperature variations or differing tension of the weigh belt. Also, because of the flowing nature of certain materials, it can be difficult to reliably keep the material on the belt, thus causing additional inaccuracy. Similar to a weigh-belt system is a weigh auger system, which operates on the same principle as a weigh belt system, but uses an auger in lieu of a belt for dispensing product.
The weigh-belt and weigh-auger types of gravimetric feeding systems, therefore, while using feedback principles to offer control, have inherent limitations that can seriously impair the accuracy of these systems.
Another type of gravimetric feeding system usable for dry bulk solid and liquid materials is the loss-in-weight system. In a loss-in-weight system, the gradual decrease in weight of the entire system (including the discharge or feeding mechanism, such as an auger device or a pump, a supply vessel or dispensing bin attached to the feeding mechanism, and the material itself) caused by the constant release of material from the scale-mounted feeding mechanism is monitored and compared to an expected decrease in weight to assess performance. Unlike the weigh-belt and weigh-auger gravimetric systems, loss-in-weight systems are not affected by material adhering to or releasing from critical areas of the weighing mechanism, or a shift in the scale's "zero" or calibration, since the entire system is weighed continuously.
Preferably, the loss-in-weight feeding system is designed with a counterbalance so that the feeding mechanism is tared-out on the scale such that the scale produces a zero weight signal when the feeding mechanism is empty. Consequently, the scale only measures the weight of the material in the feeding mechanism. Knowing the size and type of the supply vessel and the feeding mechanism, the amount of weight attributable to the material in the supply vessel can be determined. In loss-in-weight systems, a control system continually (or with great frequency) monitors a signal indicating the decreasing total weight of the material, and adjusts the feed rate output to maintain an accurate rate of weight loss over time in relation to the selected feed rate (operator-input set rate). If a weight loss amount over a given time period is larger than expected as sensed by the weighing system, the control system commands the feeding mechanism to slow down. Similarly, if the weight detected becomes too high, indicating that the weight loss has been less than expected, the control system orders an increase in the material output flow rate to cause the system to catch up with the expected feed rate.
Representative control systems for loss-in-weight feeding systems are shown and described in U.S. Pat. Nos. Re. 30,967, Re. 32,101 and Re. 32,102, incorporated herein by reference, and all assigned to the assignee of the present invention, Acrison, Inc. Also, loss-in-weight feeding systems sold by the assignee of the present invention, Acrison, Inc., including microprocessor-based control systems therefor, are commercial examples of control systems for loss-in-weight feeding systems.
Depending upon the desired accuracy of feed rate, volumetric, weigh-belt/weigh-auger or loss-in-weight systems are used for feeding materials.
During standard operation, different samples of dry solid material can be of greatly varying bulk densities, even though the samples consist of the same material. In existing loss-in-weight systems, the weight of the material in the loss-in-weight system is continuously (or almost continuously) monitored. Automatic refill of the system's integral supply vessel is initiated when the monitored weight is sufficiently low and continued until the monitored weight reaches a predetermined high weight, at which point refill is stopped. If the material has a sufficiently lower bulk density than initially determined, however, the material may not be heavy enough to trigger the shut off of the automatic refill mechanism, even when the supply vessel is full because the predetermined high shut-off weight was determined based on the higher density (weight) material. Consequently, with a low bulk density material, refill continues, forcing the loss-in-weight system to remain out of gravimetric control (thereby reducing accuracy) and, perhaps, damaging the feeding system.
In other loss-in-weight systems, a material sensing probe is positioned in the supply vessel. When the probe senses a predetermined level of material, refill is shut off. See, e.g., U.S. Pat. Nos. 4,320,855, Re. 32,101, and Re. 32,102.
Loss-in-weight feeding systems generally have a control panel that displays the volume of material in its integral supply hopper. A controller determines the volume to be displayed based, in part, on a stored bulk density value for the material. When the bulk density of the actual material in the supply hopper differs from the bulk density value stored by the controller, the displayed volume is less accurate than is desired.
A relatively accurate bulk density measurement is required to control the feed rate as well. During operation, there are times when a loss-in-weight feeding system will not operate in loss-in-weight mode and will operate temporarily in a volumetric mode. For example, during refill, the weight signal is actually increasing as material is being added to the supply vessel and so the actual loss-in-weight signal cannot be used to determine the amount of material being fed. The weight of the material fed is often determined instead based on the volume of material displaced by a displacement mechanism (such as an auger) and by the density of the material. When the density employed by the controller is not accurate, the determination of the amount of material fed during such a period of time is less accurate than is desired.
At times during operation, the feeding system does not use loss-in-weight to control but rather controls based on an estimated motor speed. The controller selects a motor speed which corresponds to the set rate by interpolating from the known feed rate for a given motor speed. This is called the "fast start" routine. For example, when the feeding system is first started or when the operator inputs a new, different set rate, the system cannot operate in a loss-in-weight mode because there are insufficient weight readings at the new set rate to accurately measure the material flow and properly control the feeding system. Therefore, to obtain quickly an accurate feed rate at start up, the controller determines a motor speed estimate corresponding to the operator-input set rate based on the "maximum feed rate." Maximum feed rate (or "mfr") is the rate at which material flows when the motor is running at 100% of motor speed. If the maximum feed rate is 100 lbs/hr at 100% of motor speed and the set rate is 50 lbs/hr, the controller directs the motor to run at 50% of motor speed. The maximum feed rate, however, changes with the density of the material. Consequently, when the bulk density of the material used by the controller is not accurate, the feed rate in "fast start" mode is less accurate than is desired.
It is an object of this invention to provide an improved material feeding system with a material sensing probe disposed in the supply vessel for detecting material near the top of the supply vessel and stopping refill when the supply vessel is about to be overfilled.
It is a further object of this invention to provide an improved material feeding system capable of calculating the density of the material in the supply vessel.
It is a further object of this invention to provide an improved material feeding system capable of using the newly calculated density of the material to improve control of the feed rate of the feeder.
It is a further object of this invention to provide an improved material feeding system capable of accurately calculating and displaying the volume of material in the supply vessel.
It is a further object of this invention to provide an improved method of calculating the density of material located in a supply vessel.
It is a further object of this invention to provide an improved method of improving the control of a material feeding system based on the newly calculated density of the material.